1. Field of Invention
The present invention relates to a method for operating a non-ferrous smelting plant, and more particularly to a method for increasing the production amount of non-ferrous metals. In a non-ferrous smelting plant, the raw material, such as copper or nickel sulfide ores or their concentrates, together with a flux mainly composed of silica ore, are introduced into a smelting furnace, such as a flash furnace or a reverberatory furnace. The present invention is hereinafter described mainly with reference to copper smelting by means of a flash furnace.
2. Description of Related Art
The silica ore, which is fed to a copper smelting furnace, such as a flash furnace or a reverberatory furnace, supplies SiO2 that reacts with Fe in the copper ores. Precious metals such as gold and silver, contained in small amounts in the silica ore, are also recovered in the copper smelting process. The silica ore is conveyed from a mine to a smelting plant and is stored outdoors, and is then crushed in a lateral type ball mill, which is appropriate for continuous operation. Since the silica ore is a flux that smoothly advances the slagging reactions in the reaction tower of a flash furnace during the smelting of copper, it is crushed to a particle size virtually as fine as that of the copper concentrate. The particle size of the crushed silica ore is usually approximately 100 μm on average. The amount of the flux is approximately 10% relative to the copper ore but this percentage increases as a grade of the copper ore is lowered.
Meanwhile, the copper ore as a raw material has been crushed and dressed at the mine and is then conveyed to a smelting plant. The resultant concentrate is subsequently dried with a drier and is then charged into the flash furnace. Incidentally, in the past, there was a step in which flux and powdered copper ore were sintered in the copper smelting, but this is no longer done.
The flux mentioned above is dried together with the copper concentrate and is then charged into the flash furnace. This method is an ordinary one, and is described in Japanese Patent No. 3,307,444 (FIG. 1), “Shigen to Sozai” (JOURNAL OF THE MINING AND MATERIALS PROCESSING INSTITUTE OF JAPAN), 1993, Vol., 109, No. 12, “Special Edition of Non-ferrous Smelting”, “Copper Smelting In Kosaka Smelter” page 938, FIG. 1. In addition, the copper concentrate and flux are dried together also in the MI method (c.f. Special Edition of Non-Ferrous Smelting” supra, page 961, FIG. 4. In all of the smelting methods described above, the copper-ore raw material and the flux are dried and then conveyed together in the identical system.
Shigen to Sozai (JOURNAL OF THE MINING AND MATERIALS PROCESSING INSTITUTE OF JAPAN) 1998, Vol. 114, No. 7., pages 447-454, “High Intensive Operation and Increase in Productivity in Saganoseki Smelter by using a Single Flash Furnace”, discloses a method for increasing the capacity of a drier to treat the copper ore and the flux. The production amount of copper is increased by reconstructing several apparatuses as follows. Diameter of a pipe for feeding air into a rotary drier is increased. A hot-air generating furnace is changed from a lateral type to a vertical type. Heavy oil is combusted in a larger amount. The capacity of an exhaust gas fan is increased. A dust collector is re-constructed.
A ball mill is described in Japanese Unexamined Patent Publications (kokai) No. 2002-172339, No. 2006-110474, and No. Hei 5-15805. However, these publications are not related to crushing of non-ferrous metal ores or flux.
Generally speaking, in order to dry and convey the ore and the flux at a higher speed, a gas-stream drying apparatus and a belt conveyor must be completely redesigned and rebuilt, which requires a large investment cost. To redesign a drier, since such factors as the solid/gas ratio and dust collecting capacity must be taken into consideration comprehensively, a number of difficulties arise in the drier reconstruction. As a usual practice, instead of reconstructing the existing drier, an additional drying and conveying system of the copper concentrate has been constructed in parallel to an existing similar drying and conveying system. In this case, operation of a non-ferrous smelting plant is carried out by two parallel systems, and hence, the drying and conveying capacity is increased. Since the additional system is constructed while an existing system is being operated, interruption of drying and conveyance of copper concentrate is minimum. However, the two parallel drying and conveying systems are detrimental in view of complicacy, ineffective operation, excessive capacity, and large investment cost.
Meanwhile, crushing of the silica ore, which serves as a flux, can be said as a factor that impedes non-ferrous smelting as described below. Fundamental heat for drying the silica ore is the crushing heat generated by the crushing action of the ball mill. When the water content of the silica ore fed to the ball mill increases, that heat is not sufficient for drying it. Dew formation, therefore, occurs in the ball mill, so that the materials contained in the ball mill, such as silica ore particles, flux powder, balls and the like, adhere to the inner wall of the pot and become bonded thereto. The water content of the silica ore and the like cannot, therefore, be lowered. The crushing efficiency is also lowered. In the worst case, crushing is difficult to continue.
Since the drying performance of a conventional ball mill of the silica ore is unsatisfactory, the usual practice has been to mix the crushed silica ore with the copper concentrate and is then dry them again in a drier together. The dried copper concentrate and the twice dried silica ore are conveyed to and charged into a flash furnace. The water content of the silica ore is a factor that limits the amount of silica ore that can be fed. In order to increase the production amount of copper and also to cope with change of grade of a copper concentrate, an increased amount of silica ore must be employed. It can, therefore be said that the drying capacity of a ball mill had not been fully utilized heretofore, thus limiting the amount of the silica ore that could be treated in a ball mill.
A countermeasure against the circumstances described hereinabove is that a smelter of non-ferrous metal would buy previously crushed silica ore. However, 15% or more of water is added to the crushed silica ore, so as to prevent the dust generation during conveyance. As a result, when the crushed silica ore is charged into a drier of copper-concentrate, the drying load of the drier is increased corresponding to the water content of the silica ore, thus limiting the amount of copper concentrate that can be treated.